The NTSC color video standard used in the U.S. and some other countries consists of luminance (brightness) information added to two modulated color difference (chrominance) subcarriers. Since the color carriers appear in a portion of the spectrum that lies within the bandwidth of the luminance, it can be difficult to separate the luminance data from the chrominance data with a minimum of mixing.
In particular, luminance information can be mistaken for color information, leading to so-called cross-luma artifacts, in which color appears in an image where it does not belong. Conversely, chrominance information can be treated as luminance, leading to cross-color artifacts, where high-frequency black-and-white bands appear instead of solid color.
The oldest method of separating luminance (Y) from chrominance (C) in a video signal is simply to use a notch filter on the composite signal to generate Y, and a bandpass filter to extract C. However, this leads to a loss of high-frequency detail and cross-luma artifacts.
More advanced comb filtering methods have been applied. These methods rely on the fact that the color subcarrier changes phase by 180.degree. every line. Thus, by using a linear combination of chroma values between two lines (assuming that the two lines are sufficiently similar), luma information is averaged out but chroma information remains. A weighted average over multiple lines can also be used, a common scheme being (-0.25.times.previous line)+(0.5.times.current line)+(-0.25.times.next line).
However, note the caveat above about the lines to be averaged being "sufficiently similar". When two lines are not sufficiently similar, cross-luma artifacts occur at vertical transitions between high-frequency (i.e., detailed) luminance areas and between other areas such as solid colors or flat gray or white fields.
The inventors have determined that there is a need for an improved method for eliminating more of these types of artifacts in order to improve image quality. The present invention provides a method and system for achieving this end.